Because of their low probability of intercept and resistance to jamming, direct sequence pseudo noise (DSPN) and frequency hopping techniques are often used to successfully conduct secure/covert communications. For increased immunity against interception, these two techniques may be combined to produce hybrid spread spectrum (HSS) signals, i.e. those which employ a combination of frequency hopping (FH) and DSPN modulation. Unfortunately, each of these two modulation techniques customarily employs its own signal processing mechanism, that does not readily lend itself to recovery of the other form of modulation, particularly in a multi-signal type environment which may contain a variety of narrowband signals, such as AM, FM, PSK, and narrowband frequency hopping signals, all occurring simultaneously within the same operating frequency spectrum.
More particularly, a conventional mechanism utilized for (narrowband, positive signal-to-noise ratio) frequency hop detection is the channelized radiometer, containing a filter bank to isolate each of the frequencies to which the FH source may hop, with each filter being tuned to a respectively different instantaneous bandwidth or channel. Interception of a (wideband, negative signal-to-noise ratio) DSPN signal, on the other hand, normally entails the use of non-linear signal processing, such as squaring or convolution, in order to collapse the wideband spectrum down to a readily identifiable spectral line. Because of the low concentrated energy content (negative S/N) of the spread signal, the integration time required to process detected energy can be expected to be considerably longer than a frequency hop dwell time, so that recovery of an HSS signal may not be successful using conventional spread spectrum processing. Of course, complicating the problem is the fact that the signal environment being monitored for the presence of hybrid signals is not particularly `clean`. It can be expected to be cluttered with a large number of signals having a variety of modulation formats, which effectively mask the presence of hybrid signals.